I've never made an online side bet for these exact reasons. If I were to ever make one, it would likely only be with someone I know (or know well enough, say from this forum) for the simple fact that I will payout if I lose and I want a reasonable expectation that I will be paid if I win. Paying out is not an issue for me if I make a bet.
Now, irl, I've made many side/prop bets on things. I've paid out 100% of the time on those I've lost. And all but one win was paid to me voluntarily. (That other one only required one reminder and a little "motivation", but it was paid as well.)
I guess it comes down to the fact I take my word and money matters very seriously, so I always pay and always make sure I'm paid. That kind of security is difficult to come by online. Now, it's also important to recognize terms of agreements as well, I don't play the interpretation game. Part of my job includes writing binding contracts, so my agreements are literal, not fuzzy "well I meant this" garbage. If I win a bet and the agreement was to pay the winner on Wednesday, the loser won't hear from me until Thursday (if I haven't been paid yet). On the flipside, if I lost that bet, I don't expect to be hounded at 8AM on Wednesday looking for your money. I have until the end of the day (based on timezone agreement = detail oriented), if I really want to be firm about it.
Anyway, that's my take on it.